Relativistic Klystron Amplifier Research Articles

Cavity online measurements with a coaxial electrical probe in an X band integrally sealed relativistic klystron amplifier.

The electromagnetic characteristics of the input cavity with electron beams loaded are measured successfully in an X-band triaxial relativistic klystron amplifier (TKA) with a high frequency electrical probe. To solve the frequency response issue in the X band, a coaxial electrical probe is optimized and inserted obliquely into the input cavity at the designed angle so that the microwaves in the cavity can be sampled and extracted with the required coupling coefficient while the probe does not interfere with the solenoid coil, which is tightly integrated outside the TKA. Besides, the TKA is redesigned as a structure integrally sealed by the solenoid coil, and the secondary sealing induced by probe insertion is implemented on the flange of the solenoid coils rather than on the wall of the TKA, which is conducive to simplifying the coupling structure while maintaining the high vacuum level. The design is examined in a GW level TKA experiment, indicating that microwaves with a power of about 670MW are generated in the input cavity at a frequency of 8.40GHz, which is completely consistent with the injected and output microwave frequencies.

Coherent Combining of Phase-Steerable High Power Microwaves Generated by Two X-Band Triaxial Klystron Amplifiers with Pulsed Magnetic Fields

We report the first experimental demonstration of coherent combining of phase-steerable high power microwaves (HPMs) generated by X-band relativistic triaxial klystron amplifier modules under the guidance of pulsed magnetic fields. Electronically agile manipulation of the HPM phase is achieved with a mean discrepancy of 4° at the gain level of 110dB, and the coherent combining efficiency has reached as high as 98.4%, leading to combined radiations with equivalent peak power of 4.3GW and average pulse duration of 112ns. The underlying phase-steering mechanism during the nonlinear beam-wave interaction process is furthermore explored by particle-in-cell simulation and theoretical analysis. This Letter paves the way for high power phase array in large scale and may stimulate new interest in research of phase-steerable high power masers.

Design and experimental research of a compact directional coupler for X-band relativistic triaxial klystron amplifier.

To realize online measurement of power, frequency, and phase for a modularized relativistic triaxial klystron amplifier in a narrow space, a compact directional coupler with high directivity, high power handling capacity, and large bandwidth has been investigated numerically and experimentally. The method with coupling holes distributed in both the longitudinal and lateral directions is proposed, and the simulation results indicate that when two holes are employed, directivity larger than 20 dB is achieved over a 300MHz bandwidth in the X-band and the maximal electric field strength is controlled to be 198.5kV/cm with 1 GW power injection, while the whole coupler longitude is optimized to be as short as 50mm. High power experiments have been performed with a 10GHz relativistic triaxial klystron amplifier, and the results show that the measured power, frequency, and phase shift of the directional coupler have good agreement with the results obtained in the far-field.

A RepetitiveKu-Band Coaxial Relativistic Klystron Amplifier Packaged With Permanent Magnets

A compact Ku-band coaxial relativistic klystron amplifier (RKA) with permanent magnets is proposed for the sake of producing repetitive high-power microwaves (HPMs) in this article. The improved input and output mode converters are designed to load the input signal at the end of this device to ensure the integrity of permanent magnets. Besides, the intense relativistic electron beam (IREB) is changed to be collected on the outer collector, which makes collector cooling feasible in the proposed device. The typical experimental results show that the Ku-band HPM with a power of 540 MW is generated, corresponding to a gain of 48 dB and an efficiency of 29%, when the guiding magnetic field is around 0.45 T produced by the permanent magnets. The result of continuous operation for 30 s at a repetition rate of 35 Hz indicates that the proposed coaxial RKA has the capability of operating at a high repetition rate.

Stable Operation of a Repetitively Pulsed X-Band Gigawatt Long Pulse Coaxial Multi-Beam Relativistic Klystron Amplifier

In this letter, the stability of an X band gigawatt coaxial multi-beam relativistic klystron amplifier (CMB-RKA) operating in repetitively pulsed mode was investigated by the particle-in-cell (PIC) simulation and experiments. A four-gap extended-interaction output cavity was optimized to avoid electrons reflection. And a water-cooled electron beams collector which was integrated with the output mode converter was designed to reduce the plasma and secondary electrons. The explosive emission conditions were improved by using the C–C composite material cathode, and the titanium beam-wave interaction structure was used to replace the stainless steel structure. All these factors brought greater stability of the CMB-RKA. In the experiment, the X-band CMB-RKA can operate at a 20 pps repetition rate with duration of 120 seconds, when the output power was about 0.95 GW with pulse width of 160 ns. The long-term stability of the X-band CMB-RKA performances during <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{5}}$ </tex-math></inline-formula> pulses was realized, which is driven by a long pulse compact accelerator.

Excitation and suppression of asymmetric modes in a coaxial relativistic klystron amplifier with cascaded single-gap bunching cavities

The excitation and suppression of asymmetric modes in a coaxial relativistic klystron amplifier (RKA) with cascaded single-gap bunching cavities are analyzed in this paper. Through theoretical analysis and simulated verification, we find that the asymmetric modes of cascaded single-gap bunching cavities have low external quality factors, the same resonant frequencies, and negative beam-loading conductance ratios so that they are easily excited by the electron beam. To solve this issue, a feasible method is proposed in this paper to suppress the asymmetric modes of cascaded single-gap bunching cavities, under the conditions of decreasing their coupling coefficients and increasing the frequency separation, choosing an appropriate drift tube length between them. These improved bunching cavities are further examined in an X-band coaxial RKA by 3D particle-in-cell simulation, which shows that high power microwaves with a power of 0.8 GW are generated corresponding to an efficiency of 40%. Furthermore, there is no asymmetric mode competition during 150 ns of simulation time.

Short-Length, High-Efficiency S-Band Coaxial Cavity Relativistic Multibeam Klystron Amplifier for Potential High Power Microwave Application

This letter reports the successful development of an S-band short-length, high-efficiency, coaxial cavity multi-beam relativistic klystron amplifier (CCMB-RKA). The simulation in one-dimensional genetic algorithm, capable of giving high-efficiency parameters, predicted 67% efficiency of CCMB-RKA, taking the interaction length as 600 mm, beam voltage as 420 kV, and beam current as 230 A. However, for accurate results, the CCMB-RKA was simulated again using three-dimensional code CHIPIC, taking RF input power, beam voltage, and beam current as 4.4 kW, 420 kV, and 3.22 kA ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$= {14} \times {230}$ </tex-math></inline-formula> A, 14 being the beam number), respectively, predicting the output power, gain, and efficiency of the device as 810 MW, 53 dB, and 60%, respectively. Finally, the designed CCMB-RKA was built and experimented with. Through 20 groups of the comparative experiments, the GW-level RF output power of the device, typically, 1.027 GW (peak), 60 ns pulse was demonstrated with 55% efficiency, 370 phase jitter, and 53 dB gain, taking the RF input power, beam voltage, and beam current as 4.8 kW, 453 kV, and 4.1 kA, respectively. The results establish the potential of the CCMB-RKA developed for high-power microwave applications.

A compact coaxial relativistic klystron amplifier with three cascaded single-gap bunching cavities for efficient output

A compact coaxial relativistic klystron amplifier (RKA) with three cascaded bunching cavities is investigated to obtain high efficiency output in this paper. When the injection power and drift tube length are both decreased for the compactness of the initial coaxial RKA, the conversion efficiency of this device decreases to only 20%, which is mainly due to the insufficient modulation of intense relativistic electron beam (IREB). To solve this issue, three cascaded bunching cavities are designed to strengthen the modulation of IREB, and each bunching cavity is designed to a non-uniform single-gap cavity with a high external quality factor. Moreover, a specific drift tube length is chosen for minimizing the TEM mode leakage between every two adjacent cavities. With these methods, the self-oscillation between input and three cascaded bunching cavities can be successfully suppressed without loading any reflectors. The proposed three cascaded single-gap bunching cavities are further examined in a Ku-band coaxial RKA, and the output efficiency of this device increases to 44%, which is more than twice that of the initial coaxial RKA. Furthermore, the injection power and axial length of the propagating IREB in the improved coaxial RKA are reduced to only 5 kW and 10 wavelengths, respectively.

Coherent Combination of Power in Space With Two X-Band Gigawatt Coaxial Multi-Beam Relativistic Klystron Amplifiers

The power capacity of an individual high power microwave (HPM) generator is severely limited by internal electric field breakdown, and the problem becomes more serious for long pulse and high frequency bands. One of the promising solutions to overcome this limitation is coherent combination of power in space. To realize coherent combination of power in space, the HPM generators must operate at the same frequency and stable phase differences. We presented an improved X-band gigawatt coaxial multi-beam relativistic klystron amplifier (CMB-RKA) with high phase stability. Two double gap buncher cavities were designed to improve the beam wave interaction efficiency, and decrease the distance for the modulated intense relativistic electron beam to reach its maximum, which significantly decrease the phase jitter due to the fluctuation of electron beam voltage and improve the phase stability of the CMB-RKA. Experimentally, the two X-band gigawatt CMB-RKA demonstrated the capability of coherent combination of power in space, typically of the combined efficiency of about 95% with the combined pulse width of 150 ns.

Investigation on High-Efficiency Beam-Wave Interaction for Coaxial Multi-Beam Relativistic Klystron Amplifier

To significantly improve the electronic efficiency of coaxial multi-beam relativistic klystron amplifier (CMB-RKA), the physical process of beam-wave interaction and parameters that affect efficiency was studied. First, the high efficiency of beam-wave interaction was discussed by simulating the efficiency versus the parameters (frequency of cavity, drift tube length between cavities, and external quality factor of output cavity), in the one-dimensional (1-D) large-signal simulation software. Moreover, the further physical process of beam-wave interaction was analyzed through simulating the current modulation factor and the number of particles at the entrance of the output cavity, in the three-dimensional (3-D) particle in cell simulation software. Last, with the optimal parameters in 3-D simulations, the CMB-RKA, which has 14 electron beams with a total current of 4.2 kA (14 × 300 A), can generate an output power of 1.02 GW with a saturation gain of 55.6 dB and an efficiency of 48.7%, when beam voltage is 500 kV, which indicated the CMB-RKA can achieve high efficiency for high-power microwave radiation.

Modular Integration of a Compact Ku-Band Relativistic Triaxial Klystron Amplifier Packaged With Permanent Magnets for High-Power Microwave Generation

To achieve modular integration of relativistic triaxial klystron amplifier (TKA) devices and reduce the energy consumption of the whole high-power microwave (HPM) system, a compact Ku-band TKA packaged with permanent magnets is proposed and investigated in this paper. By optimizing the electromagnetic structure and utilizing TEM mode energy coupling, the length of the uniform magnetic field area required for the Ku-band TKA is reduced from 32 cm to 19.6 cm, with a reduction rate of 39%. By utilizing the NdFeB-N50M mainly magnetized in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> direction, a permanent magnet guiding system weighing 90 kg is proposed. Besides, the length of the uniform zone is 20 cm with a longitudinal magnetic field of 0.5 T. Verified by the PIC simulation software CHPIC, an HPM with power of 430 MW and frequency of 14.25 GHz is generated when the diode voltage, current, and the injected microwave power are 340 kV, 4.2 kA, and 15 kW, respectively. The phase jitter of the output microwave is controlled within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\pm } 5{^\circ }$ </tex-math></inline-formula> , which is beneficial to the coherent power combination of modular integrated TKA devices

Demonstration of a Ka-Band Oversized Coaxial Multi-Beam Relativistic Klystron Amplifier for High Power Millimeter-Wave Radiation

We proposed an oversized coaxial multi-beam relativistic klystron amplifier (OCMB-RKA) with a new input coupler of lesser complexity giving enhanced input microwave power absorption rate for generating high power millimeter-wave radiation in Ka-band. The high-frequency structure of the OCMB-RKA was designed, fabricated, and cold-tested for results that agreed well with the simulated results. Furthermore, using the 3D particle-in-cell (PIC) code, the beam-wave interaction of the OCMB-RKA was simulated, considering two different materials (stainless steel and copper) making the interaction structure. When a 500 kV, 3 kA electron beam, and a focusing magnetic flux intensity of 1 T were adopted in the PIC simulation, a five-cavity OCMB-RKA yielded the output power of 610 MW at 30.525 GHz with the electronic efficiency of 40.7% and the saturated gain of 55.3 dB. Experimentally, the OCMB-RKA demonstrated its capability of generating high power Ka-band radiation typically of 390 MW power with the pulse width of 27 ns.

Efficiency Enhancement of a High Power Radial-Line Relativistic Klystron Amplifier Driven by Disk Intense Electron Beam

The power conversion efficiency of relativistic klystron amplifiers (RKAs) is severely limited by the strong space-charge repulsion force caused by the intense electron beam. A radial-line RKA (RL-RKA) driven by the disk beam may provide great potentials to alleviate this issue due to its radially propagating characteristic of the electron beam. However, such potential advantage of high efficiency has not been fully exploited in previous research due to the insufficient beam modulation and relatively low beam-wave coupling strength. In this article, an initial RL-RKA is improved significantly by utilizing a three-stage cascaded bunching link, a four-gap output cavity, and a lengthened drift tube. The three-stage cascaded bunching link plays a role in increasing the fundamental harmonic current and decreasing the velocity spread of the electron beam simultaneously. The four-gap output cavity is designed to improve the coupling impedance and strengthen the beam-wave power conversion. Moreover, a drift tube is lengthened suitably to boost the fundamental harmonic current further. Eventually, the power conversion efficiency of the RL-RKA is enhanced up to 67% by these measures, which indicates a significant advance over the initial state of 38%.

Design of a large-radius high impedance intense current diode based on gradient magnetic field

In order to reduce the size of high impedance diodes and magnetic energy consumptions, a high impedance intense current diode is designed in this paper. By reducing the center radius of the cathode and utilizing the gradient magnetic field, the electron beam is emitted in the cathode with a small radius and transmitted in the drift tube with a large radius. The influence of magnetic field distribution on the electron beam transmission is analyzed by particle-in-cell simulation. When the diode voltage is 680 kV, the impedance is 77 Ω, and the cathode radius is 60 mm, the anode radius of the traditional diode is 116 mm and the cathode–anode axial gap is 57 mm. At the same voltage and impedance, the cathode radius of the novel diode is 38 mm, the anode radius is reduced to 80 mm, and the cathode–anode axial gap is shortened to 36 mm. Further application of the novel diode to a relativistic triaxial klystron amplifier device shows that the output power of the novel diode is the same as that of the traditional diode. Through simple quantitative analysis, it is found that the magnetic energy consumption of the novel diode is less than that of the traditional diode. To sum up, the novel diode can achieve good performances in a smaller volume than the traditional diodes.

Optimal design and experimental research of several-gigawatt multiple electron beam diode

The relativistic klystron amplifier (RKA) is one of the most efficient sources to amplify a high-power microwave signal due to its intrinsic merit of high-power conversion efficiency, high gain and stable operating frequency. However, the transverse dimensions of the RKA dramatically decrease when the operating frequency increases to X band, and the power capacity of the RKA is limited by the transverse dimensions. An X-band multiple-beam relativistic klystron amplifier is proposed to overcome the radiation power limitation. Each electron beam propagates in separate drift tubes and shares the same coaxial interaction cavities in the multiple-beam relativistic klystron amplifier, and the transverse dimensions of the multiple-beam relativistic klystron amplifier are free from the operating frequency restriction and a microwave power of over 1 GW is generated in the experiment. For a high-power electron device, the transmission of electron beam is critical, and the power conversion efficiency of the device is affected. In this paper, we conduct an investigation into the transmission process of the intense relativistic multiple electron beams, and the number of the multiple electron beams is set to be 16. It is found that when the multiple electron beam is transmitted in the device, the electron beam rotates around the center of the whole device, causing the electron beam to deviate from the drift tube channel. At the same time, each electron beam rotates around itself, and the cross section of the electron beam is deformed and expanded. In the improper design of electron beam and drift tube parameters, two kinds of rotating motions cause beam to lose. A multiple-electron-beam diode structure is optimized by the particle-in-cell simulation to reduce beam loss, with the effects of the related factors taken into account. Each pole of the cathodes is made up of graphite and stainless steel. The cathode head is made up of graphite, for the graphite has a lower emission threshold. The cathode base and cathode pole are made up of stainless steel, for the stainless steel has a higher emission threshold. Also the shape and structure of cathode pole, cathode head and anode are optimized to reduce the electric field intensity on the cathode pole and enhance the electric field intensity on the end face of cathode head. At the same time, the electric field distribution of the cathode head is uniform to improve the electron beam emission uniformity. The simulation results demonstrate that the transmission efficiency of multiple electron beams can reach 99%. In the experiment, the transmission efficiency of multiple electron beams is 92% with a beam voltage and beam current of 801 kV and 9.3 kA, respectively.

Design and preliminary experiment of a disk-beam relativistic klystron amplifier for Ku-band long-pulse high power microwave radiation

Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.

An improved X-band relativistic triaxial klystron amplifier with active suppression of asymmetric TM mode self-excitation

The self-excitation of asymmetric modes is one of the main factors limiting the long-pulse operation of the relativistic triaxial klystron amplifier (TKA). Research studies have revealed that the asymmetric TM modes with negative beam loading conductance (Ge/G0) in the bunching cavities are the main culprits of self-excitation. The existing studies focus on passively destroying the growth of asymmetric TM modes, but we attempt to actively suppress their self-excitation in this paper. Investigation results demonstrate that the employment of single-gap bunching cavities can significantly reduce the risk of self-excitation of asymmetric TM modes, as the Ge/G0 of asymmetric TMn11 modes in single period structures are positive. The current modulation capacity of a single-gap bunching cavity, however, is not sufficient, so a cascade structure consisting of two groups of bunching cavities is employed to compensate for this deficiency. Based on the above conclusions, an X-band TKA device with four single-gap beam–wave interaction cavities is proposed and investigated in this paper. The three-dimensional particle-in-cell simulation results demonstrate that there is no self-excitation of asymmetric modes in the proposed X-band TKA device.

Output microwave characteristics of a Ka-band relativistic klystron amplifier with spatially nonuniform multibeam electron emission

The effects of spatial nonuniformity of electron multibeam emission on the output characteristics of a coaxial multibeam Ka-band relativistic klystron amplifier (RKA) are investigated using particle-in-cell simulations. The spatial nonuniformity is simulated using four different electron multibeam emission models, with (1) a continuous cathode non-emission area, (2) discontinuous cathode emission areas, (3) discontinuous cathode emission areas with different electron emission current densities, and (4) cathode emission areas with different and time-varying electron emission current densities. The simulation results with the first model show that the output microwave power of the RKA decreases as the continuous non-emission area increases, but that size of the continuous non-emission area has little effect on the frequency and pulse width of the RKA. The results with the second model show that the more uniform the electron current density in the discontinuous emission areas, the greater the output microwave power. The results with the third model show that the more uniform the distribution of the Z component of the induced electromagnetic field in the discontinuous emission areas, the greater the output microwave power, and also that the output microwave power increases as this Z component increases. The results with the fourth model show that fluctuations in the electron current density in the discontinuous emission areas in the steady-state phase of RKA operation cause fluctuations in both microwave frequency and output microwave power and that these fluctuations increase as the fluctuations in electron current density become stronger and as they become more rapid.

Influence of annular electron beam uniformities on the microwave characteristics in an S-band relativistic klystron amplifier

The non-uniform annular electron beam models are established based on the annular cathode explosive emission luminescence images. The influence of electron beam uniformities on the microwave characteristics of an annular structure S-band relativistic klystron amplifier output cavity is investigated with particle-in-cell simulations. The electron beam non-uniformities are simulated using four different electron emission models: (1) continuous area without emission, (2) spaced emission, (3) enhanced emission, and (4) current density variation emission. The simulation results with the first emission model show that the output power decreases as the continuous area without emission increases, while the continuous area without emission has little effect on the frequency and pulse width. The simulation results with the second emission model show that the output power of the spaced emission is related to the distribution of non-emission areas. The more evenly the areas without emission are distributed on the annular cathode, the greater the output power is. The simulation results with the third emission model show that the higher the current density in the local area is, the smaller the output power is. The simulation results with the fourth emission model show that the fluctuation of current density causes the fluctuation of frequency and output power. The larger the change area of the current density is, the greater the fluctuation amplitudes of the output power and frequency are. The larger the fluctuation amplitude of current density is, the greater the fluctuation amplitudes of the output power and frequency are.

Design and Optimization of Reflectors in a Relativistic Triaxial Klystron Amplifier

The inevitable power leakage of the TEM mode in relativistic triaxial klystron amplifier (TKA) devices will cause energy coupling between different beam-wave interaction cavities, which could interfere or even damage the stable operation of the devices. Specially designed mode reflectors have been proposed to suppress the power leakage of the TEM mode in TKA devices. The design and optimization methods of the reflectors have been investigated systematically in this article. The research results show that a reflector is essentially a high-order mode cavity, and the frequency of this mode is close to the operation frequency of the devices. Furthermore, the effects of reflector loading on the cavities are investigated and compound modulation of multiple cavities with reflectors is examined. The simulation results demonstrate that the loading of downstream cavities will aggravate the energy coupling between the upstream cavities. To achieve the isolation of cavities in multireflector devices, the eigenfrequencies of the two reflectors in front of the bunching cavities should be separated as far as possible. With the employment of three carefully designed reflectors, the power leakage of the TEM mode is controlled within an acceptable range and no parasitic frequency self-excitation is observed inside a proposed Ku-band TKA device.